Patterns in tree species richness as a test of the glacial extinction hypothesis

Abstract

IT is well established that Europe has far fewer tree species and genera than either eastern North America or eastern Asia^1–3. Fossil evidence shows that west-central Europe had a much richer tree flora during the Upper Tertiary (25–2 Myr BP), with many genera which now survive only in temperate regions of North America and Asia³. These trees seem to have been eliminated from Europe during cold, dry glacial periods of the Pleistocene Era (2–0.001 Myr BP). Their failure to recolonize may be due to a failure of species to reach potential refugia during the glacial phase^2,3. Alternatively, present-day climates, rather than purely historical effects of extinction or suvival in refugia, may explain the observed contrasts in richness between regions. Currie and Paquin⁴ showed that the number of tree species in Britain and Ireland corresponds to that of North American areas of similar climate, without the need to invoke the historical explanation. Here we report that the differences in species richness between three northern temperate regions, Europe, eastern North America and eastern Asia, can be mainly explained in terms of present-day climatic factors. Similarly, and despite its long period of isolation from the northern temperate flora, the tree flora of New Zealand corresponds closely to the same relationship between climate and species richness. By contrast, greater species richness (compared with Europe and North America) found in some parts of eastern Asia cannot readily be accounted for in terms of present climate, and may be due to lower rates of extinction during glacial phases.